This invention relates to crossfire tubes extending between adjacent combustors in a land-based gas turbine.
The annular arrangement of combustors in a stationary, or land-based gas turbine with interconnecting crossfire tubes is generally well known as disclosed in, for example, commonly owned U.S. Pat. No. 4,249,372. As disclosed in the ""372 patent, a typical cross ignition assembly comprises tubular members extending between aligned openings in adjacent combustors that are held in place by means that position the opposite ends of the tubular members or crossfire tubes in fluid communication with the adjacent combustion chambers. The purpose of the crossfire tubes is to provide for the ignition of fuel in one combustion chamber from ignited fuel in an adjacent combustion chamber, thereby eliminating the need for a separate igniter in each combustor. Specifically, when chamber to chamber crossfire is desired, it is accomplished by a pressure pulse of hot gases transferring from a firing chamber to an unfired chamber through the crossfire tube. The crossfire tubes also serve the purpose of equalizing to some extent the pressures between combustion chambers.
Current crossfire tube design includes a plurality of purge air holes (usually six) arranged about the circumference of the crossfire tube, approximately midway along its axial length. An arrangement of this type is disclosed in commonly owned U.S. Pat. No. 5,896,742. Purge air is fed to the crossfire tube purge air holes at approximately the compressor discharge pressure so as to prevent unwanted migration of oil (unburned fuel) between adjacent combustors. This purge air, however, opposes the cross-firing pressure pulse, and can actually prevent firing of the adjacent combustor. Specifically, the purge air feed pressure and flow rate both inherently resist the crossfire pressure pulse. Inhibiting oil intrusion, however, is controlled by the purge air flow rate combined with the convergence of the tube shape. Thus, a minimum discharge flow rate out of the crossfire tubes is necessary to inhibit oil intrusion into the crossfire tube. Oil intrusion is a source of auto ignition, which, in turn, is a cause of failure of the gas turbine to correctly operate. Accordingly, the purge air flow rate must be sufficient to inhibit oil intrusion, but the combined pressure and flow rate must not be too great to keep a pressure pulse of hot gases from transferring from one combustion chamber to the other through the crossfire tube. Compressor discharge pressure and the crossfire tube purge air hole size set the flow rate inside the crossfire tubes. Attempts to balance the crossfire performance and resistance to oil intrusion by varying the hole size, however, have not been successful. It has been found that the feed pressure and flow rate are either too high to consistently achieve good crossfire or are too low to inhibit oil intrusion.
Thus, the problem to be solved is that the flow rate cannot be decreased to improve crossfire performance without risking an increase of oil intrusion, and possibly other harmful effects elsewhere in the combustion turbine.
Some model gas turbines have reduced the purge air feed pressure to the crossfire tubes by mechanical blockage for reasons divorced from crossfire tube performance. These gas turbines have good crossfire performance but, since they do not operate on oil, they do not have the same design constraints with respect to oil intrusion into the crossfire tubes.
In accordance with this invention, both purge air feed pressure to the crossfire tube and the purge air flow rate within the crossfire tube are separately affected by creating a pressure drop mechanism upstream of the purge air feed holes into the crossfire tube. This is accomplished by creating an integral chamber as part of the crossfire tube assembly that will operate such that during crossfire, the purge air is temporarily xe2x80x9cstalledxe2x80x9d in the integral chamber. This is made possible by reducing the purge feed pressure such that the re-light pulse from the firing chamber to the unfired chamber can overcome the pressure drop across the purge feed holes.
Thus, in accordance with the broader aspects of the invention, there is provided a crossfire tube for connecting adjacent combustors in a gas turbine, the crossfire tube comprising a hollow tubular body having opposite end portions adapted to be secured to the adjacent combustors; an annular chamber surrounding a mid-section of the hollow tubular member; a first plurality of purge air holes in an outer wall of the chamber, and a second plurality of purge air holes in an inner wall of the chamber opening into the crossfire tube.
In another aspect, the invention provides a crossfire tube assembly for connecting adjacent combustors in a gas turbine, the crossfire tube assembly comprising a hollow tubular body having opposite end portions and a plurality of purge air feed holes arranged in a circumferential array about a mid-section of the hollow tubular body and adapted to feed purge air into the hollow tubular body; the hollow tubular body having means for reducing compressor discharge air pressure prior to entry into the hollow tubular body.
In still another aspect, the invention relates to a method of supplying purge air to a crossfire tube located between a pair of adjacent combustors in a gas turbine, the crossfire tube including a hollow tubular body adapted for connection between the adjacent combustors, the method comprising establishing a chamber about the tubular body; utilizing compressor discharge air as crossfire tube purge air; feeding the purge air at a first pressure approximately equal to the compressor discharge air pressure into the chamber to thereby reduce the pressure to a second, lower pressure; and subsequently feeding the purge air from the chamber into the tubular body of the crossfire tube at the lower pressure.